Foaming in hydrocarbon processes is usually undesirable as foaming typically adversely affects hydrocarbon process efficiencies. One example of a hydrocarbon process that can be acutely affected by undesirable foaming is coking.
Coking is one of the older refining processes. The purpose of a coke plant is to convert heavy residual oils (e.g. tar, asphalt, etc.) into lighter, more valuable motor fuel blending stocks. Refinery coking is controlled, severe, thermal cracking. It is a process in which the high molecular weight hydrocarbon residue (normally from the bottoms of the vacuum flasher in a refinery crude unit) are cracked or broken up into smaller and more valuable hydrocarbons.
Coking is accomplished by subjecting the feed charge to an extreme temperature of approximately 950° F. that initiates the cracking process. The light hydrocarbons formed as a result of the cracking process flash off and are separated in conventional fractionating equipment. The material that is left behind after cracking is coke, which is almost pure carbon. In addition to coke, which is of value in the metal industry in the manufacture of electrodes, fuel coke, titanium dioxide, etc., the products of a coke plant include gas (refinery fuel and LPG), unstabilized (wild) gasoline, light gas oil, and heavy gas oil.
The lion's share of the world's coking capacity is represented by delayed coking processes. Delayed coking can be thought of as a continuous batch reaction. The process makes use of paired coke drums. One drum (the active drum) is used as a reaction vessel for the thermal cracking of residual oils. This active drum slowly fills with coke as the cracking process proceeds. While the active drum is being filled with coke, a second drum (the inactive drum) is in the process of having coke removed from it. The coke drums are sized so that by the time the active drum is filled with coke, the inactive drum is empty. The process flow is then switched to the empty drum, which becomes the active drum. The full drum becomes the inactive drum and is emptied or decoked. By switching the process flow back and forth between the two drums in this way, the coking operation can continue uninterrupted.
After being heated in a direct-fired furnace, the oil is charged to the bottom of the active coke drum. The cracked light hydrocarbons rise to the top of the drum where they are removed and charged to a fractionator for separation. The heavier hydrocarbons are left behind, and the retained heat causes them to crack to coke.
One problem frequently encountered in coke production is foaming in the coke drums. In coking processes, foam formation is the result of evolved gas molecules in a liquid. Foaming is a function of many variables including surface tension, pressure, viscosity, and other properties of the gas/liquid system. While foaming in aqueous systems has been studied extensively, relatively little is known about controlling foaming in organic systems. This foaming problem is particularly acute in the later portions of a fill cycle or when a coke drum is depressured before coking is completed. Foaming is especially problematic because of the possibility of carry-over which can result in plugged overhead lines and lost profit opportunity to clean the lines. Foaming in coke drums also reduces the useable space in the drums for coke capacity, ultimately limiting total production capacity.
Conventional approaches for addressing foaming in coke drums suffer from a variety of significant disadvantages. One approach for dealing with foaming is to simply decrease production rates to limit foaming. This approach is obviously disadvantageous in that overall production is reduced. The temperature can also be increased, but this shortens the run length of the coker furnace because the fouling rate increases in the furnace tubes, unfortunately resulting in more frequent downtime to clean the furnace tubes.
Other approaches include injecting a silicone-based compound (e.g. polydimethylsiloxane compounds) to reduce foaming. These silicon-based compounds when used in excess have been known to poison downstream hydrotreating catalysts. Catalyst poisoning is a severe problem as the catalyst cannot be regenerated. Indeed, poisoned catalyst must be replaced offline, which requires a costly shutdown of the hydrotreating facility as well as possibly other units in the refinery. The initial silicon-based compounds are known to decompose and lose effectiveness over time, thus limiting their effectiveness and resulting in waste due to the required constant addition of the compounds during key portions of the coking cycle.
The problem of foaming in coke drums can be detected if appropriate indicators are available. By the time foaming is detected, however, it may be too late to prevent undesirable carryover or plugging of overhead lines if action is not taken quickly.